The invention relates to the application of bus bars for interconnecting solar cells, and in particular to thermal spray depositing of bus bars across the grid line surfaces and the back surfaces of solar cells to form interconnections for a sequence of solar cells to make a solar panel.
The prior art and practice for making the connection between solar cells is generally by the use of wires or tabs to connect the bus bars of the front side of one cell to the metallized back side of the next cell, although the detailed procedures employed by different manufacturers might vary. A typical tabbing process for connecting solar cells in series is described below.
By using a specially designed tabbing and stringing machine, a metallized solar cell is laid on two parallel copper tabs that are coated with a silver-tin alloy. These two tabs are soldered to the back of the cell through two rows of silver pads that are previously screen-printed on the back surface of the cell. The reason for using silver pads is that the connecting tabs cannot be soldered on an aluminum surface directly, which is the material used for the metallization of the back surface.
Another two metal tabs, each about twice the length of a cell, are then placed on the front surface of the first cell with one end covering each screen-printed bus bar. This end of the tabs is soldered to the printed bus bars. The other end of each of the two metal tabs is then bent over the end edge of the solar cell to the level of the backside surface of the next cell, extending over the intended position for the next cell, and are then cut off at about the full length of the next cell.
The next cell is then laid backside down on the extended tab ends. The steps described in the last two paragraphs are repeated for this and consecutive cells until the desired number of cells are linked to form a string or row of cells in the panel. A solar panel is constructed by linking one or several rows of such cells into an array. Solar panels commonly use arrays ranging, for example, from 1 row of 9 cells to 4 rows of 9 cells. Other sizes are possible.
In the prior art process, the tabbing machine is very complex, combining mechanical parts that move and cut tabs and electrical parts that heat up the cell surface and solder the tabs. Moreover, for the soldering of tabs on the back side, an extra step is required for screen-printing the silver pads. This adds to the cost of machines and working space, as well as to production time and costs, since extra machines for printing and baking are necessary.
Referring now to another area of art, thermal spray is a relatively mature technology for making coating layers. This technology uses hot jet methods (now cold jet methods are also available) to accelerate and melt or soften powder particles that are fed into the jet. The high speed particles or droplets deposit on a surface and form the desired coating. Application of thermal spray technology has been explored in last two decades especially in the area of amorphous silicon solar cell manufacturing; see Janowiecki et al""s U.S. Pat. No. 4,003,770, published Jan. 18, 1977.
Plasma spraying for this technology was mainly used for thin film silicon deposition on glass or other substrates. The use of arc plasma spray for deposition of metal contacts on silicon substrates has also been reported: see Lindmayer""s U.S. Pat. No. 4,240,842 published Dec. 23, 1980, U.S. Pat. No. 4,297,391 published Oct. 27, 1981, and U.S. Pat. No. 4,331,703 published May 25, 1982; and Narasimhan et al""s U.S. Pat. No. 4,320,251 published Mar. 16, 1982.
In summary, there is room for improvement in the setup and production costs and efficiency with which solar panels are constructed from solar cells, such as might be achieved by novel combinations and adaptations of existing and new techniques.
The basic concept of this invention resides in a new application of well established thermal spray technology, for the fabrication or application of conductive paths and connections directly to a single- or poly-crystalline silicon solar cell and of interconnections between cells.
It is an object of this invention to provide a method for making bus bars on solar cells which link the grid lines or contact fingers on the front surface. It is a further object of this invention to use the same method to extend the deposited bus bar from the top surface of one cell onto a backing or base material for interconnecting a next sequential solar cell, and so on, to form a connected row of cells as for a solar panel.
It is a still further the object of this invention to make the interconnection of cells at much faster and economical manner than is the prior art practice. Another object is to provide a method of interconnection of cells that is of much better electrical and mechanical quality compared to prior art practices, which leads to improved solar/electric conversion efficiencies. It is a yet further the object to enable the interconnection of solar cells by a single apparatus instead of multiple-step, tabbing and stringing equipment as is currently practiced.
Other and numerous objects and advantages will be apparent from the figures, description of preferred embodiments, abstract, and claims that follow.